This invention relates to a method and apparatus for deforming thermoplastic material and, more specifically, to a method and apparatus for heating lengths of thermoplastic material without heating the ends of said lengths.
The typical deformation process for thermoplastic tubing and pipe and other items of elongated thermoplastic material requires that the tubing be heated at the section where deformation is desired. Once heated, the tubing may be readily bent or deformed on a jig or a form or other mechanical device and then cooled and set.
Prior heating methods have included totally immersing the tubing in heated "oil" baths. However, deformation is often desired in only a section of the tubing, such as in the manufacture of elbows for use in electrical conduit, in which bends are desired in the middle of the tubing while the ends remain unbent, retaining their generally cylindrical shape in order to permit standard connectors to fit over the ends. It is then desirable that only the section of the tubing to be deformed be heated, while the ends remain relatively cool. With this and related methods, the bath was substantially at a uniform temperature and thus the entire length was heated. This method was frequently undesirable since the entire length of tubing, including the ends, became susceptible to sags and bends and other types of deformation.
Other prior heating methods have included heating the tubing under an infrared lamp and simultaneously rolling the tubing to make heating uniform and to avoid undesirable deformation. This process had the advantage of heating only sections of the tubing while the ends could remain unheated, but it was disadvantageous for hollow thermoplastic tubing or pipe in that only the outside of the tubing or pipe was heated directly. It would be much more advantageous to heat pipe or tubing from both the inside and the outside, since the process would then be speeded up greatly. Furthermore, if the tubing was left too long under the infrared lamp, the plastic was likely to be burned.
Still other prior processes for heating tubing have included spraying the tubing with hot "oil" and simultaneously rolling the tubing. While this rendered the tubing less susceptible to burn damage, it was still a slow process since, again, only the outside was heated directly.